The present invention relates to a radiological imaging apparatus comprising a semiconductor radiation detector which detects incident radiation to output a detection signal for it.
Semiconductor radiation detectors using Si, Ge, or CdTe collect both plural electrons and holes resulting from ionization in the detector substance caused by radiation to generate an electric signal (generally a voltage pulse). The semiconductor radiation detectors thus obtain information on the energy and incident timing of the radiation, the number of incident radiations, and the position of incidence.
Migration of charges in the semiconductor generates signal induced charges at a signal read electrode. Accordingly, if the charges are captured or recoupled owing to the level of impurities in the semiconductor, the charges do not contribute to the radiation signal having generated the charges. The phenomenon in which the signal becomes detective by the capture or recoupling is called a capture defect.
In a semiconductor radiation detector using a compound semiconductor device such as CdTe or HgI2, the mobility of holes is generally lower than that of electrons. Consequently, the time required to collect all the holes is insufficiently short compared to the lifetime of the holes before capture. When a long time is required to collect all the holes, a more serious capture defect occurs depending on the lifetime of the holes.
Since an anode collects the electrons, while a cathode collects the holes. Accordingly, when a radiation reaction position is close to the anode, the holes migrate a long distance before collection (a long time is required to collect the holes). As a result, the capture defect is most significant. In contrast, when the radiation reaction position is close to the cathode, the electrons must migrate a long distance before collection. However, owing to their high mobility, almost no capture defect occurs in the electrons. Further, only an insignificant capture defect occurs in the holes because they move only a short distance before collection.
Accordingly, the output signal varies depending on the interaction position of the radiation in the semiconductor. Different output signals thus result from the same input energy. This is a major cause of degradation of energy resolution.
To avoid the degradation of the energy resolution, a method corrects the capture defect by connecting two shaping amplifiers to the semiconductor radiation detector and using output signals from these shaping amplifiers for one radiation incidence event. That is, one of the shaping amplifiers has a shaping time that is insufficient for the charge collection time, to intentionally generate what is called a ballistic defect that is a signal defect correlated markedly with the capture defect. Thus, the pulse height values of the output signals from the shaping amplifiers are corrected for each event (see, for example, JP-A-61-14590 (Page 5, FIG. 1). This makes it possible to obtain a high energy resolution with a low applied voltage resulting in a significant capture defect, for a certain combination of mobility, lifetime, and anode-cathode distance.
JP-A-61-14591 (FIGS. 2(a) and 2(b)) describes a radiological imaging apparatus in which a plurality of semiconductor radiation detectors are arranged in a plurality of columns and a plurality of rows. In each column, the semiconductor radiation detectors contained in the column have first electrodes connected together using first interconnects. In each row, semiconductor radiation detectors contained in the column have second electrodes connected together using second interconnects. A shaping amplifier is connected to each of the plurality of first interconnects and the plurality of second interconnects. Outputs from these shaping amplifiers are input to a concurrency determining device. The outputs from the shaping amplifiers connected to the first interconnects are input to a main amplifier including an integrator and an amplifier to shape waveform and an auxiliary amplifier including a differentiator to shape waveform (the time constant of the auxiliary amplifier is smaller than that of the main amplifier). The radiological imaging apparatus described in JP-A-61-14591 compares the pulse heights of output signals from the main and auxiliary amplifiers to estimate a ballistic defect. The apparatus then uses the ballistic defect to correct the pulse height of the output signal from the main amplifier.
However, if semiconductor radiation detectors are installed in an actual apparatus, for example, a radiological imaging apparatus, it is indispensable to densely mount the semiconductor radiation detectors because of the need for both high positional resolution and high sensitivity. This makes it difficult to provide sufficient intervals between the electrodes of adjacent semiconductor radiation detectors. The inventors have thus found that this configuration may pose a new problem, that is, the occurrence of a large parasitic capacity. The occurrence of a large parasitic capacity degrades the characteristics of a process for shaping the waveform of waves from the shaping amplifiers connected to the electrodes. This precludes correction of the pulse height values of radiation detection signals.